Various proposals have been made, in order to achieve effective printing on a substrate, by causing a change of colour in the substrate on which the printing is to appear. Various pigments have been proposed, which can be used to mark a substrate on the application of laser energy. Some of these proposals may be found in, for example, WO00/43456, JP-A-11001065, EP-A-0522370, EP-A-0797511, U.S. Pat. No. 5,053,440, U.S. Pat. No. 5,350,792 (a plastics moulding composition comprising a polyoxymethylene and animal charcoal), U.S. Pat. No. 5,928,780, U.S. Pat. No. 6,017,972 and U.S. Pat. No. 6,019,831. U.S. Pat. No. 5,489,639 and U.S. Pat. No. 5,884,079 disclose that copper hydroxy phosphate is a laser-markable material.
Ammonium octamolybdate, having the formula (NH4)4Mo8O26 and abbreviated herein as AOM, is a readily available material that has fire-retardant properties. The particular utility of AOM and related compounds as laser-markable materials is disclosed in WO02/07458. As also disclosed there, AOM is readily available, and is selective for a robust, low-power CO2 laser operating at about 10,600 nm.